High-power fiber lasers have received a wide attention in the past ten years. Such lasers with several kilowatts (kWs) or several tens of kWs have been used as commercially available products in industries. In comparison with solid-state lasers, fiber lasers have a unique feature of a superb beam quality at high power due to an all-fiber configuration. That is, all the optical components used in the fiber lasers are of an optical fiber type, connected using fusion splices without air interfaces between any two of the optical components in connection. The optical components include multiple diode laser pumps with multiple optical fiber pigtails, an amplification optical fiber with two fiber Bragg gratings, a transmission fiber spliced to the amplification optical fiber, and an optical fiber combiner with multiple input optical fibers to splice to the multiple optical fiber pigtails of the multiple diode laser pumps and with an output optical fiber to splice to the amplification optical fiber. The amplification optical fiber, doped with a rare earth element such as erbium (Er) or ytterbium (Yb) as a gain medium, provides for a beneficial geometry and a large surface to volume ratio, thus allowing for extraordinary heat dispersion and reducing thermal lensing effect when compared to rod type solid state lasers. The amplification optical fiber with the gain medium receives and absorbs optical energy from the multiple diode laser pumps through the optical fiber combiner and creates a coherent laser light via a resonator built by using the two fiber Bragg gratings at two ends of the amplification optical fiber. Such multimode fiber lasers in the 2- to 6-kW regime are ideal for cutting and welding, and particularly in the area of materials processing and laser machining as a reliable replacement for bulky diode pumped solid-state lasers and CO2 lasers. It has been shown that lengthening the amplification optical fiber can inherently increase power of the fiber lasers without a limit. However, double clad optical fibers (DCOFs) used in both the output optical fiber of the optical fiber combiner and the amplification optical fiber are surrounded by a polymer coating with a limited tolerance to heat. In other words, the maximum thermal load provided by the coating dictates the maximum output power that the fiber laser can attain.
Not similar to optical fibers used in optical communications, where the coatings outside the optical fibers simply play a role of mechanical protection, the polymer coatings used in DCOFs perform both mechanical and optical functions. DCOFs use dual acrylate coatings, with a first low refractive index polymer coating in contact with the glass core, and with a durable second coating to protect the first relatively soft low refractive index coating. In other words, the second coating mechanically protects the low refractive index coating from mechanical chips, cuts, or scratches which may result in optical energy to leak out from the fiber, possibly creating localized hot spots or catastrophic burns at high pump powers. DCOFs with the dual acrylate coating can pass the stringent reliability test specified by Telcodia GR-20 standard used in the telecom industry. Without doubt, DCOFs with the dual acrylate coating have a high tensile strength of greater than 700 kilo-pounds per square inch (kpsi) and an exceptional stress corrosion resistance. However, according to the GR-20 standard, after exposing DCOFs to an environment of 85° C. and 85% relative humidity (RH) for 720 hours, it shows an excess loss for laser output power due to possible degradation of the low refractive index coating in exposure to temperature and humidity. It is noted that the 85° C./85% condition not only affects the optical reliability of the low refractive index coating but also causes OH ingression into the glass core of the optical fiber, increasing attenuation of the glass core. For example, the attenuation in the typical pump wavelength range is well below a negligible 0.01 dB/m. After exposure the optical fiber to temperature and humidity, either wavelength-dependent or independent attenuation increases. The attenuation, in general, is associated with OH ingression in the silica, glass defects formed due to moisture ingression, and light scattered by the low refractive index polymer. That is, during the 85° C./85% RH test, moisture not only degrades the low-index polymer but also penetrates the glass cladding, resulting in the excess fiber loss.
In high-power laser delivery applications, a laser light or optical energy from a fiber laser is delivered to an application area using a transmission optical fiber or a delivery optical fiber. In the application area, the laser light must exit from an optical fiber end to free space. In order to maintain laser beam quality and prevent optical components upstream and downstream from damaging, a residual pump power, an amplified spontaneous emission (ASE) power, or an undesired signal power captured in the cladding of a DCOF in an optical fiber laser or an optical fiber amplifier needs to be removed. The residual pump power can be in hundreds of watts in kW fiber lasers, and the ASE can be in the range of several tens of watts, typically much higher in a fiber amplifier. A conventional way to remove the cladding light is to remove the low refractive-index fluoroacrylic coating for a length of the optical fiber and re-coat with a high refractive-index coating. Such an optical component is called cladding mode stripper. In this approach, a high numerical aperture (NA) cladding light will be more effectively stripped than a low-NA cladding light. However, this approach remains very useful because the low-NA pump light in the cladding is, in fact, more strongly absorbed in a fiber laser, leaving more of the high-NA light in the residual pump in the cladding. One real issue is that the act of cladding stripping is occurred over just several millimeters, creating a localized hot spot, which needs to be thermally managed to prevent the cladding mode stripper from damaging. Several different cladding mode strippers with longitudinal variations have been proposed to improve hot spot localization but to suffer from increased complexity.
Surfaces substantially perpendicular to a laser propagating direction are vulnerable to the high power-density laser light because any of surface imperfections such as impurity, defects, and contamination and Fresnel reflections due to a refractive index change when the laser light moves between two media can lower a damage threshold of the surfaces, easily burning down the surface. That is one of reasons that a fiber laser system is so popular over a solid-state laser nowadays because the possible number of surfaces is minimized by splicing all the optical fiber components in the system together, thus significantly increasing system's reliability. In many fiber laser applications, the laser light must exit from an optical fiber to free space. With a coreless piece of optical fiber, so called optical end cap, placed between the output end of the fiber laser and the free space, the risk of damage at the end face can be dramatically reduced. In this case, the beam expands along the coreless end cap, thereby reducing an optical power density at the end face and also back reflection due to the divergence in the beam.
As mentioned above, the optical end cap sustainable at high power is critical for highly reliable high-power fiber lasers. In the high-power fiber lasers, an integrated water-cooled package has been proposed, in which an optical end cap is completely immersed in the circulating water for efficient cooling. In this case, however, OH ingression in the silica and glass defects generated from moisture ingression can reduce the reliability of such an optical end cap. It is, therefore, the purpose of this patent application to disclose several water-cooled schemes in packaging an optical fiber head without immersing a delivery optical fiber in cooling water. The advantages include un-deteriorated attenuation and unstressed core and cladding of the delivery optical fiber, which shows long-term reliability and unaffected speckle patterns and beam quality when a high-power laser light is delivered to free space.